1. Field of the Invention
The present invention relates to optical communications systems. More particularly, the present invention relates to polarization independent optical amplifiers. Still more particularly, the present invention relates to a method for packaging semiconductor optical amplifiers that eliminates polarization dependence.
2. Description of the Background Art
Optical communications systems are rapidly becoming a widespread and dominant technology in telecommunications and networking. Optical communications systems transmit information optically at very high speeds over fiber optics. The basic components of an optical communications system include: an optical transmitter; fiber optics, and an optical receiver. An optical transmitter incorporates information to be communicated into an optical signal and sends the optical signal. Fiber optics carries the optical signal over a distance. Finally, the optical receiver receives the optical signal and recovers the information therein.
A particular problem in optical communications is attenuation of the optical signal. The attenuation may occur due to transmission of the signal, distribution of the signal, and losses due to insertion of components in the transmission path. Optical amplifiers are used to compensate for signal attenuation. One conventional type of optical amplifier is a Semiconductor Optical Amplifier (SOA). Conventional semiconductor optical amplifiers comprise a semiconductor laser like structure that operates below the lasing threshold. Typically, an electrical current is used to pump the electronic population in the active region of the amplifier. The optical signal is input from fiber optics to the active region of the amplifier, experiences gain due to stimulated emission as it passes through the active region, and is output in amplified form to further fiber optics. Such a conventional SOA is shown in FIG. 1.
Polarization dependence/sensitivity (TE/TM differential gain) is another problem for optical networks. Light can be divided into two polarizations, and in an optical fiber, it varies randomly between these polarizations with time. If a device in an optical communication system has polarization dependent output properties such as polarization dependent gain or loss, then the output signal power can vary with time. This causes problems in the network because it causes signal levels to change with time in an uncontrollable manner, particularly when several of these devices are cascaded in series.
One particular problem with SOAs is that they are inherently polarization sensitive asymmetrical structures. The optical fiber imparts cylindrical coordinates that are perfectly symmetric and does not have significant polarization dependence. However, SOA waveguides generally have a planar geometry such as a rectangular cross-section 0.5 microns by 3 microns, in addition to having actual material gain that can be different for the two polarizations. Thus, it is very difficult to eliminate polarization dependent gain (loss) from SOAs.
There have been attempts in the prior art to reduce and eliminate the polarization dependence of SOAs. In particular, the prior art has attempted to reduce the polarization sensitivity with on-chip techniques that change the structure of the SOA. However, these attempts at an on-chip solution are difficult to manufacture and have had limited success. It is very difficult to correct for polarization dependent gain because of the very precise thickness control and precise strain control required when manufacturing SOAs. For example, SOAs may require that strain be controlled within +/xe2x88x920.2 percentage to achieve +/xe2x88x920.5 dB polarization dependence (See, Bart H. Verbeek, Leo H. Spiekmann, Talk CThG3, Conference on Lasers and Electro-optics, San Francisco, Calif., May 7-12, 2000). This corresponds to requiring that material composition be controlled within about 0.1%. The prior art is not able to control the semiconductor processing steps to the extent necessary to produce SOAs having less than 1 dB polarization sensitivity. Furthermore, the SOAs need to be polarization insensitive at all wavelengths and at all operating currents and that adds additional complexity to the design and semiconductor processing. For the present application, polarization insensitive is defined to be having polarization sensitivity of less than 1 dB.
Thus, there is a need for a system and method for manufacturing polarization insensitive SOAs.
The problems and disadvantages heretofore associated with the prior art are overcome by the present invention. The present invention is an optical amplifier module that is insensitive to polarization. The optical amplifier module of the present invention preferably comprises a first fiber, a second fiber, a semiconductor optical amplifier (SOA), and a polarization dependent loss (PDL) unit. In a preferred embodiment, the first fiber provides for optical input, and is optically coupled to the SOA. The SOA amplifies the optical signal received from the first fiber and outputs the amplified signal. The output of the SOA in turn is optically coupled to the PDL unit. The PDL unit provides polarization dependent loss and the loss is preferably selected to match the polarization dependent gain of the SOA such that when two are coupled there is no polarization dependence. In other words, the loss added by the PDL unit makes the gain from input to the SOA to output by the PDL unit the same for both polarizations. The output of the PDL is optically coupled to the fiber for transmission output. In one exemplary embodiment, the PDL unit is a predetermined length of fiber, at a predetermined rotational position, made from a predetermined material, with a known polarization dependent loss such that the polarization dependent loss in the fiber matches the polarization dependent gain of the SOA.
The present invention also comprises a method for manufacturing an optical amplifier module that is polarization insensitive. The method comprises the steps of: determining the polarization dependent gain of an SOA, determining the polarization dependent loss of a plurality of PDL units, selecting a PDL unit such that the overall polarization dependent loss when coupled to the SOA is reduced, and packaging the SOA and the PDL unit as an optical amplifier module.